1. Field of the Invention
The present invention relates to an LED array head and a minute reflection optical elements array for use in the LED array head. In particular, the present invention relates to an LED array head used as a light source of an LED array printer and a minute reflection optical elements array for use in the LED array head. The printer includes an optical writing apparatus, such as a LED array printer utilizing an electrophotographic process. The present invention further relates to a method of making the LED array head and the minute reflection optical elements array.
2. Discussion of the Background
The LED array printer using an electrophotographic process with an LED array head as its light source has a merit that deformation of the optical system due to vibration or heat is very small compared with a laser printer using a laser raster system.
FIG. 4 shows an outline of a basic structure of such an LED array printer. In FIG. 4, the LED array printer is constructed so that light rays emitted from an LED array head 100 are focused on a surface of a photosensitive body 102 (e.g., a photosensitive drum) through an equal-scale focusing optical system 101. Thereby, the photosensitive surface of the photosensitive body is exposed by the focused light rays and an electrostatic latent image is formed thereon.
One example of the LED array head 100, which has already been proposed, is shown in FIG. 5. In FIG. 5, an array of LED light-emitting portions 111 are formed so as to protrude from a surface of a silicon base plate 110 by causing the chemical compound semiconductor to grow in the low-cost silicon base plate 110 used as a head base plate (see Japanese Laid-open Patent Publication No. 9-45955/1997).
In the LED array 100, respective LED light-emitting portions 111 include plural semiconductor layers electrically connected to individual electrodes 112 and a common electrode 113 both formed on the silicon base plate 110. The light emitted from the respective LED light-emitting portions 111 is focused on the photosensitive body 102 through the equal-scale focusing optical system 101.
As shown in FIG. 6, the light-emitting portion 111 is formed so as to protrude from the silicon base plate 110. Thus, light is radiated directly towards the equal-scale optical system 101. However, light is also radiated from a side surface of the respective LED light-emitting portions 111 in a direction of a surface of the base plate (hereinafter referred to as xe2x80x9cside surface lightxe2x80x9d). This radiated light is reflected by the respective LED light-emitting portions 111, the individual electrodes 112, the common electrode 113, or by a bonding wire for the individual electrodes 112. The light reflected is radiated toward the photosensitive body 102 as light-emitting points. Therefore, a latent image is formed on the photosensitive body 102 and which includes the side surface light (i.e., a noise component). After the latent image is developed, the developed image is transferred onto paper as a toner image. Consequently, there is a deterioration of image quality, such as an unevenness of line width on the image.
For example, background art Japanese Laid-open Patent Publication No. 6-25244/1994 has already proposed that a light-intercepting film be provided on side surface portions of the respective LED light-emitting portions 111. The light-emitting portions 111 are formed to have a protruding shape by growing and laminating (four-layer) semiconductor layers: 114a, 114b, 114c and 114d on the silicon base plate 110 (see FIG. 6). The electric current flows from the individual electrodes 112 to the common electrode 113. The individual electrodes 112 and common electrode 113 are respectively formed on upper and lower surfaces of the silicon base plate 110. Thereby, the light-emitting portions 111 emit light. In this example, the essentially desired light to be emitted is emitted from the upper surface 111a of the respective LED light-emitting portions 111. However, a side surface 111b of the LED light-emitting portion 111 also emits light. Thus, this side-surface light is suppressed by forming a reflective film (i.e., light intercepting film) with a protection film 115. The proposed LED array head of the background art as mentioned above adopts such a structure.
However, a problem exists with this structure because a part of the light is transmitted through a reflection film 116. Even though this transmitted light is faint, the light exerts a negative influence on an exposure pattern on a surface of the photosensitive body 102. Therefore, it is necessary to use a thin film compound of material having a sufficient effect of intercepting the light as the reflection film 116 (e.g., metal) instead of a transparent material such as an oxide compound (oxided substance).
Here, as the method of forming the metal reflection film, a material having an anisotropic property in an film forming direction is generally used. As shown in FIG. 6, the shape of the side surface of the LED light-emitting portion 111 in the array direction thereof is constricted in a shape of a 90xc2x0xe2x80x94inclined letter V, xe2x80x9c less than xe2x80x9d, or in the shape of a reversed mesa. Therefore, if the side surface 111b of the LED light-emitting portion 111 is vertical or has an overhang of a reversed-mesa shape it is inevitably necessary to adopt a film forming process of inclining and piling the base plate, etc. Thus, the process of assembling the LED array head is complicated. This is a problem to be solved.
Furthermore, to avoid a short-circuit between the conductive metal electrodes, it is necessary to insulate the individual electrodes and the light intercepting film with an insulating film. Thus, the number of processes inevitably increases.
In addition, radiated light has a property of a wide radiation angle. Therefore, it is difficult to effectively direct the radiated light into the equal-size focusing optical system 101 in FIG. 4. In addition, the essential light is first directed into the equal-size focusing optical system 101 and then radiated to expose the photosensitive body 102. Therefore, it is desired to narrow the radiation angle thereof so that the light can be easily directed into the equal size-focusing optical system as incident light, and thereby a light combining efficiency can be improved.
In recent years, personal and office information apparatus have been largely developed. Therefore, a demand for a high resolution, compact, and inexpensive apparatus has been considerably raised, such as a printer using an electrophotographic process. As an apparatus satisfying such a demand, for instance, there exists an LED array printer using an LED array head having a large number of very small (miniature-sized) LED light-emitting elements arranged in a state of an array on a base plate. Since an LED array printer is a solid-state scanning type printer using the LED array head as the writing light source, it is possible to easily make the apparatus small-sized, compared the raster scanner type laser printer utilizing the semiconductor laser. Furthermore, because the respective LED light-emitting elements in the LED array head perform the writing operation in parallel, it is possible to have high-speed output, especially compared with the raster scanner type laser printer. The LED array printer has the merits as mentioned above.
Another example of an LED array head has been proposed in Japanese Laid-open Patent Publication No. 6-125,1114/1994. For example, referring to FIG. 10, the reference numeral 201 represents an LED array chip, in which an LED light-emitting element 202, a bonding pad 203, etc. are formed. The LED array chip is mounted on a base plate 204 together with a driver IC (not shown), etc. The LED array chip 201 is constructed such that an electric current is supplied so as to flow between the bonding pad 203 on the LED array chip 201 and a bonding pad 205 disposed on a surface of the base plate via a bonding wire 206 connected therebetween.
In addition, the LED light-emitting element 202 is connected to the bonding pad 203 via a wiring pattern not shown. The light emitted from the LED light-emitting element 202 In an LED array head 207 is focused and radiated onto the surface of a photosensitive body 209 (e.g., drum) through an equal-size focusing optical system 208. Thereby, the photosensitive surface is exposed by the focused and radiated light and an electrostatic latent image is formed thereon. In general, as the equal-size focusing optical system 208, a lens array having a refractive index distribution type fiber is used.
However, in the LED array head 207, the light directed into the equal-size focusing optical system 208 includes incident light from areas excluding the essential light-emitting area. Such undesirable light results in the deterioration of the image quality. For instance, a part of the emitted light is reflected by the bonding wire 206. This reflected light enters into the area of visual field xcex8 of the equal-size focusing optical system 208. Thus, the light is radiated as unnecessary light on a light spot on the surface of the photosensitive body 209 or as vacillating light. As a result, the image quality is lowered. In addition, sometimes areas of the base plate 204, in which light should not be emitted from, emit light due to an abnormal state of a process at a time of creating light emission. Concerning this point, the LED array printer using the LED array head as the writing light source has a relatively low resolution, e.g., 400 dpi. Therefore, the influence exerted by such vacillating light is not such a serious problem. However, in recent years, a high resolution apparatus (e.g.,600 through 1,200 dpi) has been realized. Therefore, it is impossible to disregard the above-mentioned matters as one of the reasons of lowering the image quality.
As a method of preventing the lowering of the image quality due to the light radiated from areas excluding that of the LED light emitting element, the Japanese Laid-open Patent Publication No. 4-179,1558/1992, has proposed a method of coating all of the areas excluding that of the LED light-emitting element with paints exhibiting a light intercepting effect. If such method is applied also for the bonding wire, light reflected by the bonding wire can be reduced.
However, because a surface of the light intercepting paints has some amount of a reflection coefficient, the reflected light on this position cannot be completely eliminated. As discussed above, for a background LED array printer having low resolution, this is not such a serious problem. However, in the LED array printer having a high resolution developed in recent years, even a small amount of unnecessarily reflected light exerts a negative influence on the image. Therefore, the image quality is inevitably lowered. The method of preventing the negative influence due to this reflected light by use of the light intercepting paints is insufficient for solving such a problem arising in the recent high-resolution LED array printer.
Furthermore, even though the above method is somewhat effective, there is a limitation in a weather-proof characteristic of the coatings (e.g., paints) used. Namely, when the LED array printer is used for a long time, the surface of the light intercepting paints is deteriorated, which results in a rough surface. Consequently, the surface of the paints becomes white to some extent. As a result, the reflection coefficient gradually increases, and thereby the effect initially desired cannot be obtained.
One object of the present invention is to solve the above-mentioned problems.
Another object of the present invention is to provide an optical apparatus using an LED array head capable of solving the above-mentioned problems.
Yet another object of the present invention is provide an LED array head which suppresses the negative influence caused by side surface light emitted from side surfaces of the protruding LED light-emitting portions and to narrow the radiation angle of essential light emitted from the upper surfaces of the LED light-emitting portions. Thus, the efficiency of combining light with the equal-size focusing optical system is improved.
Still another object of the present invention is to provide a minute reflection optical element array, preferable for a high resolution writing operation performed by an image forming apparatus.
Another object of the present invention is to provide a minute reflection optical element array, preferable for a high resolution writing operation, without lowering an image quality due to light emitted from areas other than an essential light emitting area.
These and other objects are achieved by providing an LED array head which includes a base plate, and an array of protruding LED light-emitting portions formed on the base plate. Further, a mirror array including reflection mirror structures is formed on an upper circumferential surface of corresponding LED light-emitting portions, thereby narrowing a radiation angle of light emitted by the corresponding light-emitting portions. In addition, the reflection mirror structures have a circular cone frustum shape with a diameter at an upper emission side larger than a diameter at a lower emission side. Also provided is a minute reflection optical elements array of an optical writing apparatus having an array of light-emitting elements formed on a base plate of an LED array head. The optical elements array includes opening portions having a tapering cross-section shape which becomes narrower in a direction toward the base plate at sides opposing corresponding light-emitting elements. In addition, the corresponding opening portions are disposed over the corresponding light-emitting elements. The minute reflection optical elements array also includes marginal escape recess portions formed at a surface of the optical elements array to be joined with a surface of the base plate of the array of light-emitting elements. The marginal escape recess portions have a shape corresponding to a shape of a wiring portion on the surface of the array of light-emitting elements at sides of the corresponding light-emitting elements. Also provided is a method of making the devices described above.